Support RawMessageDelivery on SNS subscriptions

Hi All,

I noticed that the AWS SDK did not support [RawMessageDelivery](http://docs.aws.amazon.com/sns/latest/dg/large-payload-raw-message.html) for SNS.

To fix this, I tried adding it analogous to the delivery policy code and re-factored that part a bit.

Please tell if you'd like me to change anything or add more or different test, etc.